This research is aimed at uncovering the cellular mechanisms underlying behavioral choice and associative learning (active and passive avoidance conditioning) at the level of the single neuron. Experiments are performed on a relatively simple invertebrate, the gastropod mollusk Pleurobranchaea californica. Our research has five broad aims. First we are studying reciprocal inhibition on the behavioral level between two independent behavioral acts, feeding and withdrawal from tactile stimulation. Second, we are analyzing the neural mechanisms subserving this reciprocal inhibition. Third, we are analyzing the cellular effects of a behaviorally active polypeptide hormone, egg-laying hormone. Fourth, we are studying chemosensory transduction, with the aim of finding out how Pleurobranchaea defects and distinguishes different food substances. This study is expected to provide us clues about the mechanisms of learning in this animal. Fifth, since both choice and learning involve the feedng behavior, we are performing detailed neurophysiological investigations of the central motor network that controls feeding, using conventional electrophysiology and labeling of functionally active nerve cells with radioactive deoxyglucose. The research has health-related significance to learning mechanisms and disabilities, and to polypeptide hormone disorders such as diabetes.